How can I scale and extrude a mesh proportionally?
Separate off the first tier of seating (seat and back) into their own object.  Add a new empty at the origin.



Select the seating object and add an array modifier with a fixed count of say 15 and clear the relative offset checkbox.  Use object offset with the object set to the Empty.  Check the Merge checkbox.

In front orthographic view, move the Empty up on Z until the seating rows are correctly aligned.  In top orthographic view, scale the Empty until the tiers are correctly aligned.



Apply the array modifier, join back to the original object and remove doubles.  You should have a clean mesh.